Method for making a photomask assembly incorporating a porous frame

ABSTRACT

A photomask assembly is described having a frame for supporting a transparent pellicle above a photomask substrate, defining a closed pellicle space overlaying the substrate. The frame is formed of a porous material configured to allow the pellicle space to be purged with an inert gas within a reasonable processing time period, thereby removing any harmful chemicals that might be present. The frame preferably is made by a method that includes preparing a gel by a sol-gel process, drying the gel, and partially densifying the dry gel. The resulting frame has a gas permeability to oxygen or nitrogen higher than about 10 ml.mm/cm 2 .min.MPa, an average pore size between 0.001 micrometer and 10 micrometers, and a coefficient of thermal expansion between 0.01 ppm/° C. and 10 ppm/° C.

Priority is claimed from pending U.S. Provisional Patent ApplicationSer. No. 60/415,598, filed Oct. 2, 2002, and Ser. No. 60/415,732, filedOct. 2, 2002, incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates generally to photomask assemblies used in alithographic process and, more particularly, to photomask assembliesincorporating porous frames, such frames being configured to facilitatepurging of the space adjacent to the photomask substrate. This inventionalso relates to methods for making such photomask assemblies.

In the semiconductor industry, intricate patterns of electronic chipsare generally made using photolithographic processes. These processesutilize photomask assemblies, in combination with laser exposuresystems, to transfer patterns onto electronic chips. FIG. 1 shows thecomponents of a conventional photomask assembly 10, including a pellicle12, a frame 14, antireflective films 16, a liquid coating 18, aphotomask substrate 20, mounting adhesive 22, a cover adhesive 24, glue26, and a release liner 28.

The photomask substrate 20 typically is made of synthetic silica and isprinted with a pattern of an electronic circuit or chip (not shown inFIG. 1) to be produced. The pellicle 12 typically is made of a softtransparent polymer, and it functions to protect the patterned surfaceof the photomask substrate from outside contaminants, thereby extendingthe substrate's lifetime and decreasing the production costs of theelectronic chips. Both surfaces of the pellicle are coated withantireflective films 16, to increase the transmittance of the laserlight. The frame 14 typically is made of anodized aluminum, and itfunctions to support the polymer pellicle above the substrate so as todefine a pellicle space 32. The frame typically has a rectangular shape,although it alternatively can have a polygonal, oval, or circular shape.The frame and pellicle typically are mounted on the photomask substrateusing any of a variety of adhesives. The release liner 28 typically ismade of a polymer material, and it functions to facilitate an easyremoval of the pellicle, to allow various components of the assembly tobe cleaned or replaced. The liquid coating 18 is applied to the innersurface of the frame, to capture particulate matter present in thepellicle space 32.

The pattern on the photomask substrate 20 is repetitively transferredonto the surface of a succession of electronic chips (not shown inFIG. 1) by continuously exposing the photomask substrate to light of aspecified wavelength. Conventional photolithography systems use laserlight sources operating at wavelengths of 436 nm, 365 nm, 248 nm, andmore recently 193 nm. In general, lower wavelengths provide a finerpattern resolution. It is expected that 157 nm lasers will be used inthe future to develop even finer patterns.

The exposure to light from high energy lasers operating at suchultraviolet (UV) and deep-ultraviolet (DUV) wavelengths can heat theassembly and trigger certain undesired photochemical and thermalreactions. These reactions can cause defects to form and grow on thesurfaces of the components of the photomask assembly, eventuallydestroying the patterns transferred to the chips.

The formation and growth mechanisms resulting from the undesiredphotochemical and thermal reactions identified above are described in apublication by Bhattacharyya et al., entitled “Investigation of ReticleDefect Formation at DUV Lithography,” BACUS News, February 2003, Vol. 19(2). These mechanisms are affected by several factors, including thephotomask assembly components described above, the assembly container,the storage and fabrication environment, the exposure systemenvironment, residuals from the cleaning of the assembly components,repetitive exposure to the laser light, and the wavelength of the laser.The Bhattacharyya et al. publication reports that outgassing of ammoniafrom the frame adhesive is responsible for forming defects at 248 nmexposure. It also reports that the number of defects increasesconsiderably after the 700^(th) exposure to a 193 nm laser. Thesedefects might form due to the presence of water vapor, ammonia, carbondioxide, and sulfuric acid, which either have diffused into the pelliclespace from the outside environment and/or have been formed by degassingor degradation of the assembly components. It is believed that oxygenpresent in the exposure environment also can cause defects to be formed.In addition, oxygen and water vapor can absorb the laser light at 193nm, thereby decreasing the light transmittance. Volatile hydrocarbonsalso can contribute to the formation of defects and to a decrease intransmittance, by absorbing the laser light. Such problems are expectedto increase with the future use of higher energy, 157 nmphotolithography systems.

The formation of defects can be partially or completely-avoided bypurging the pellicle space with an inert gas such as nitrogen after theassembly has been fabricated and/or during the laser exposure. Thispurging removes the harmful chemicals mentioned above. As explained in apublication by Cullins, entitled “LITJ360-157 nm Mask Materials,”International SEMATECH's 157 nm Technical Data Review, December 2001,some incidental purging may occur through the pellicle itself, becausethe pellicle is formed of a polymer material having some permeability.However, this purging is thought to be too slow to eliminate all theproblems discussed above within a reasonable processing time. Also, itis known that the soft polymer pellicles can easily degrade whenrepetitively exposed to light from UV and DUV lasers, causingconsiderable reduction in light transmittance, particularly at 157 nm.In addition, soft polymer pellicles cannot easily be cleaned andhandled. U.S. Pat. No. 6,524,754 to Eynon suggests that hard pelliclesformed of synthetic or fused silica can be substituted for the softpolymer pellicles. Although such hard pellicles can solve the cleaning,handling, and degradation problems, they are impermeable to gases andthereby not suitable for purging through the pellicle.

Conventional photomask assemblies incorporate frames made of anodizedaluminum, which have the following significant disadvantages. First,because the aluminum frame is generally impermeable to most gases, thepellicle space cannot be purged through the frame.

Second, there is a considerable mismatch between the coefficient ofthermal expansion (CTE) of the aluminum frame and that of the photomasksubstrate. The high purity synthetic silica widely used in manufacturingof photomask substrates for conventional lithography has a CTE of about0.55 ppm/° C., which is significantly lower than that of the aluminumframe, about 25 ppm/° C. Fluorinated synthetic silica is considered tobe a material of choice for manufacturing of photomask substrates forDUV, particularly for 157 nm lithography. The CTE of the fluorinatedsilica is affected by the level of fluorine doping. For example, silicaarticles doped with about 8,000 ppm and about 15,000 ppm fluorine haveCTEs of about 0.51 ppm/° C. and about 0.43 ppm/° C. respectively. Thehard pellicles also made of silica or fluorinated silica have CTEssimilar to those of the photomask substrate. The generation of heatduring the manufacturing of the electronic chips causes the temperatureof the photomask assembly to increase above the room temperature.Because of the large CTE mismatch between the frame and the substrate,this heating generates significant stresses in the assembly. As aresult, the printed area can be distorted, degrading the imagetransferred on the chip. Also, the pellicle can bend unacceptably,further aggravating the image degradation. These problems become moreacute when DUV photolithography wavelengths, such as 193 nm and 157 nm,are used. As reported by Kikugawa et al., in a publication entitled“Current Status of Hard Pellicle Development,” International SEMATECH's157 nm Technical Data Review (December 2001), the bending of the hardpellicle was intolerable, around 50 micrometers, when the assembly isheated only from 21.6° C. to 26° C.

Third, aluminum frames cannot be machined to better than about a20-micrometer surface flatness. The sharpness of the image transferredonto the chip highly depends on the optical alignment of the pelliclefilm with respect to the substrate. Any misalignment causes opticaldistortions resulting in patterns with bad quality. This is especially avery acute problem for 157-nm lithography. It is therefore preferable tohave a frame which is made of a material suitable for grinding andpolishing to obtain surface flatness levels better than 20 micrometers.

Another problem encountered by conventional photomask assemblies resultsfrom pressure gradients that can arise between the pellicle space andthe assembly's exterior, e.g., during shipment of the assemblies usingaircraft, exposing the assemblies to varying pressures. U.S. Pat. No.4,833,051 to Imamura, U.S. Pat. No. 5,529,819 to Campi, U.S. Pat. No.5,344,677 to Hong, and U.S. Pat. No. 5,814,381 to Kuo disclose photomaskassemblies incorporating vent structures for overcoming this problem byequalizing pressure between the pellicle space and the assembly'sexterior. These vent structures, or channels, are constructed by formingchannels having sizes in the range of 50 micrometers to 2,000micrometers. These channels penetrate through the frame and/or theadhesive layers used in mounting the frame to the photomask assembly.The channels cannot by themselves prevent the diffusion of particlessmaller than 10 micrometers into the pellicle space from the assembly'sexterior, so the channels take the form of tortuous, zigzag-shapedstructures, to trap the particles. The installing of filter systems inthe channels and/or applying adhesive coatings onto walls of thechannels also are disclosed for preventing the diffusion of particles.

The frames disclosed in the Campi, Hong, and Kuo patents are made ofaluminum, stainless steel, or like which have higher CTEs than that ofthe photomask substrate and/or the hard pellicle and, therefore, cannotprevent the image degradation problems described above. Furthermore, theuse of tortuous vent structures, filters, and/or adhesives, mentionedabove, are considered to unduly complicate the construction of photomaskassemblies.

U.S. Pat. No. 6,593,034 to Shirasaki describes an aluminum, stainlesssteel, or polyethylene frame having a vent structure for use in purgingthe pellicle space with nitrogen to prevent the absorption of the laserlight by oxygen. The vent structure has 500-micrometer holes penetratingthrough the frame body, so it incorporates a filter system to preventdiffusion of particles smaller than 10 micrometers. The frames describedin Shirasaki patent have CTEs higher than that of the photomasksubstrate and/or the hard pellicle, and it, therefore, cannot preventthe image degradation problems described above. In addition, the needfor a vent structure in the frame body and the need for a filter systemare considered to unduly complicate the construction of photomaskassemblies. Furthermore, frames made of metals such as aluminum,stainless steel, or the like, generally are not suitable for cleaning byaggressive cleaning agents such as acids to remove contaminants, withoutproducing corrosion-inducing metallic contaminants.

A frame suitable for purging photomask assemblies incorporating hardpellicles is described in a publication by Cullins entitled “LITJ360-157nm Mask Materials,” International SEMATECH's 157 nm Technical DataReview (December 2001). The disclosed frame includes six holes coveredwith Gortex® filters. The material of the frame and the CTE of theframe, and the sizes of the holes, are not disclosed. The need forfilters is considered to unduly complicate manufacturing of theassembly. In addition, the presence of the filters can causecontamination due to outgassing both from the filters themselves andalso from adhesives used for mounting the filters.

S. Kikugawa et al. report on the preparation of synthetic silica framesin “Current Status of Hard Pellicle Development,” International SEMATECH's 157 nm Technical Data Review (December 2001). The publicationshows that silica frames can be machined to a surface flatness level ofless than 1 micrometer it also shows that a hard pellicle mounted on asynthetic silica frame will bend by only 0.8 micrometer when theassembly is heated from 21.6° C. to 26° C. In contrast, a hard pelliclemounted on an anodized aluminum frame will bend by as much as 50micrometers when heated over such a temperature range. This resultindicates that the CTE of the synthetic silica frame closely matchesthat of the photomask substrate and the hard pellicle. This silica framehas multiple holes, which are reported to be 1,200 micrometers indiameter, to allow purging the pellicle space with nitrogen. Since asingle particle smaller than 10 micrometers in size entering thepellicle can render the entire produced photomask useless, thismulti-hole fused silica frames are covered with filters to preventparticle contamination. However, the need for filters complicatesmanufacturing of the assembly and can cause contamination due tooutgassing from the filters and also from adhesives used for mountingthe filters. Furthermore, since the inner surface area of such frames isvery low, the frame's scavenging capability of the volatile contaminantsis considered to be only minimal.

It should be apparent that there exists a need for a photomask assemblyincorporating a frame made of a porous material having sufficientpermeability to allow for acceptable purging rates, pore sizes smallerthan 10 micrometers to prevent particles from entering the pelliclespace, and sufficient capability to scavenge contaminants, whilepossessing a CTE compatible with the photomask substrate and/or thepellicle. The present invention fulfills these needs and providesfurther related advantages.

SUMMARY OF THE INVENTION

The present invention is embodied in a photomask assembly incorporatinga porous frame, and in a method for making it, wherein the porous framehas a gas permeability to oxygen or nitrogen higher than about 10ml.mm/cm².min.MPa, an average pore size between 0.001 micrometer and 10micrometers, and a coefficient of thermal expansion between 0.01 ppm/°C. and 10 ppm/° C.

In more detailed features of the invention, the gas permeability of theporous frame to oxygen or nitrogen more preferably is higher than about40 ml.mm/cm².min.Mpa, and most preferably is higher than about 70ml.mm/cm².min.MPa.

Further, the average pore size of the porous frame more preferably isbetween 0.01 micrometer and 1 micrometer, and most preferably is between0.08 micrometer and 1 micrometer. Further, the coefficient of thermalexpansion of the porous frame is more preferably between 0.1 ppm/° C.and 1 ppm/° C., and most preferably is between 0.3 ppm/° C. and 0.7ppm/° C. In addition, the frame's coefficient of thermal expansionpreferably matches that of the photomask substrate and/or the hardpellicle to which the frame is attached within ±20%.

In other more detailed features of the invention, the surface flatnessof the porous frame preferably is less than about 20 micrometers, morepreferably is less than about 5 micrometers, and most preferably is lessthan about 1 micrometer. Further, the pore surface area of the porousframe preferably is larger than 10 m²/g, more preferably is larger than25 m²/g, and most preferably is larger than 70 m²/g. Further, theelastic modulus of the porous frame preferably is higher than 1 GPa,more preferably is higher than 5 GPa, and most preferably is higher than10 GPa. Further, the frame's modulus of rupture preferably is higherthan 1 MPa, more preferably is higher than 5 MPa, and most preferably ishigher than 10 MPa.

In addition, the porous frame preferably is configured to scavengeharmful chemicals in an amount higher than 0.01 weight percent, and morepreferably higher than 0.05 weight percent, of the material of theframe. The porous frame preferably is formed of a material selected fromthe group consisting of silica, fluorinated silica, ZrO₂, Al₂O₃,SiO₂—Al₂O₃, SiO₂—B₂O₃ and mixtures thereof, and most preferably isformed of a material selected from the group consisting of silica andfluorinated silica having a purity of greater than about 96 weightpercent silica.

In accordance with the method of the invention, the porous frame is madeby preparing a gel by a sol-gel process, drying the gel, and partiallydensifying the dry gel. The dry gel preferably comprises silica, and itis prepared using silicon alkoxide and fumed silica.

In optional, additional features, the method further can includemachining the densified dry gel to form the frame. This machining can beaccomplished by diamond tool machining, ultrasonic milling, lasermachining, or water jet machining. This machining either can machine thedensified dry gel to form the final frame or can machine the densifieddry gel to form rectangular bars that then are welded together to formthe frame. The machining preferably machines the densified dry gel toless than 20-micrometer surface flatness. Alternatively, the gel can beinitially prepared in a mold dimensioned such that, when the gel issubsequently dried and partially densified, the frame will be configuredto have desired dimensions without the need for machining.

In more detailed features of the method of the invention, the step ofpartially densifying comprises partially densifying the dry gel at aprescribed partial densification temperature in an atmosphere comprisinghelium, nitrogen, oxygen, or mixtures thereof. This partialdensification is carried out at a temperature that preferably is withina range of 650° C. to 1260° C., more preferably is within a range of1100° C. to 1200° C., and most preferably is at about 1180° C. Further,partial densification is carried out by heating the dry gel to theprescribed partial densification temperature at a rate preferablybetween 1° C./hr and 200° C./hr, more preferably between 10° C./hr and100° C./hr, and most preferably about 15° C./hr. Further, partialdensification includes maintaining the dry gel at the prescribed partialdensification temperature preferably for a duration in a range of 1 hourto 100 hours, more preferably for a duration in a range of 1 hour to 30hours, and most preferably for a duration of about 4 hours.

In other more detailed features of the method of the invention, the stepof partially densifying is carried out in an atmosphere consistingessentially of a mixture of oxygen and nitrogen or helium, the mixturehaving an oxygen concentration between 3% and 20%. More preferably, theatmosphere consists essentially of a mixture of oxygen and nitrogen orhelium, the mixture having an oxygen concentration of about 7%.

In other, optional features of the invention, the method can furtherinclude steps of removing hydrocarbons from the dry gel by heating thedry gel at a temperature between 150° C. and 300° C., and halogenatingthe dry gel using a halogenation agent at a temperature between 650° C.and 1,200° C., after the step of removing hydrocarbons. Further, themethod can further include steps of oxygenating the dry gel after thestep of halogenating, and then re-halogenating the dry gel after thestep of oxygenating.

In yet other more detailed features of the invention, the step ofpartially densifying the dry gel can further include: (1) partiallydensifying the dry gel at a prescribed initial partial densificationtemperature, (2) machining the partially densified dry gel to a desiredporous frame shape, and (3) partially densifying the porous frame at aprescribed final partial densification temperature, wherein the finalpartial densification temperature is greater than the initial partialdensification temperature by between about 50° C. and about 300° C. Theprescribed final partial densification temperature preferably is in arange of 650° C. and 1,260° C., more preferably is in a range of 1,100°C. and 1,200° C., and most preferably is about 1,180° C.

In yet other more detailed features of the invention, the step ofpartially densifying the dry gel include: (1) partially densifying thedry gel at the prescribed initial partial densification temperature, (2)machining the partially densified dry gel after the step of partiallydensifying the dry gel, to produce a desired porous frame shape, (3)annealing the machined dry gel, at an annealing temperature that rangesbetween the initial partial densification temperature and about 300° C.lower than the initial partial densification temperature, and (4)partially densifying the annealed dry gel at a prescribed final partialdensification temperature, wherein the final partial densificationtemperature is higher than the initial partial densification temperatureby between about 50° C. and about 300° C.

The preferred method of the invention, for making a porous silica framesuitable for use in a photomask assembly, includes steps of: (1)preparing a dry gel comprising more than 99.9% silica, using a sol-gelmethod, (2) halogenating the dry gel by heating the gel from about 650°C. to about 1,050° C., at a heating rate of about 25° C./hr in anatmosphere of about 33% chlorine and about 67% helium, and maintainingthe dry gel at about 1,050° C., for a duration of about 1 hour in theatmosphere, (3) partially densifying the halogenated dry gel by heatingthe halogenated dry gel from about 1,050° C. to about 1,180° C., at aheating rate of about 25° C./hr in an atmosphere of about 7% oxygen andabout 93% helium, and maintaining the halogenated dry gel at about1,180° C. for about 4 hours, and (4) machining the partially densifieddry gel into a desired frame shape having a flatness of less than 1micrometer.

Other features and advantages of the present invention should becomeapparent from the following description of the preferred embodiments andmethods, taken in conjunction with the accompanying drawings, whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of a prior art photomask assembly.

FIG. 2 is a graphical representation of the relationship between partialdensification temperature, fracture strength, and inner surface area ofdry silica gel articles partially densified at about 1140° C., about1180° C., about 1220° C., and about 1260° C. for about four hours,prepared using the method of the present invention.

FIG. 3 is a graphical representation of the relationship between partialdensification temperature and permeability of dry silica gel articles tonitrogen and oxygen. These articles were partially densified at about1140° C., about 1180° C., about 1220° C., and about 1260° C. for aboutfour hours, prepared using the method of the present invention.

FIG. 4 is a graphical representation of the relationship between partialdensification temperature and the coefficient of thermal expansion (CTE)of a dry silica gel article partially densified at about 1180° C. forabout four hours, prepared using the method of the present invention andan aluminum article.

FIG. 5 is a graphical representation of the relationship between partialdensification temperature and the ethanol scavenging capability of drysilica gel articles partially densified at about 1140° C., about 1180°C., about 1220° C., and about 1260° C. for about four hours, preparedusing the method of the present

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a photomask assembly incorporating aporous frame and to processes for making such a frame. Porous framesembodying the invention should meet multiple physical specifications,including gas permeability, average pore size, coefficient of thermalexpansion (CTE), surface flatness, mechanical strength, inner surfacearea, and scavenging capability.

The porous frame should have high gas permeability, to allow thepellicle space to be purged with an inert gas such as nitrogen within areasonable processing time period, to partially or completely remove anyharmful chemicals present. These chemicals can be present in thepellicle space either because they are generated within the space orbecause they have diffused into the space. These chemicals include watervapor, oxygen, volatile hydrocarbons, ammonia, carbon dioxide, andsulfuric acid. To remove the chemicals, the permeability of the porousframe to nitrogen or oxygen preferably is higher than 10ml.mm/cm².min.MPa, more preferably is higher than 40 ml.mm/cm².min.MPa,and most preferably is higher than 70 ml.mm/cm².min.MPa. This high gaspermeability can be achieved by configuring the frame to include poreshaving average pore sizes larger than 0.001 micrometer, more preferablylarger than 0.01 micrometer, and most preferably larger than 0.08micrometer. To prevent particles present in the environment surroundingthe photomask assembly from migrating into the pellicle space, theaverage pore size of the frame preferably is less than 10 micrometersand most preferably is less than 1 micrometer. Thus, the average poresize of the frame preferably is in the range of 0.001 micrometer to 10micrometers, more preferably is in the range of 0.01 micrometer and 1micrometer, and most preferably is in the range of 0.08 micrometer to 1micrometer.

The CTE of the porous frame should closely match that of the photomasksubstrate and/or that of an overlying hard pellicle, to minimizedistortion of the pattern on the photomask substrate and the pelliclecaused by heating of the assembly during the manufacturing of electronicchips. The CTE of the frame preferably is in the range of 0.01 ppm/° C.to 10 ppm/° C., more preferably is in the range of 0.1 ppm/° C. to 1ppm/° C., and most preferably is in the range of 0.3 ppm/° C. to 0.7ppm/° C. The most preferable material for the porous frame is a materialhaving a CTE value that closely matches that of the photomask substrateand/or the hard pellicle, with a difference of ±20%.

It should be possible to machine the porous frame to a level of flatnessthat minimizes the optical distortions that might be caused bymisalignment of the hard pellicle with respect to the patterns on thephotomask substrate. To obtain proper alignment, the porous frame shouldbe machined to have flatness preferably less than 20 micrometers, morepreferably less than 5 micrometers, and most preferably less than 1micrometer.

The porous frame should have sufficient fracture strength to withstandmanual handling or stresses that might arise during the frame'spreparation, including its processing and incorporation into thephotomask assembly, and during the manufacturing of the electronic chip.To achieve this requisite strength, the elastic modulus of the porousframe preferably is higher than 1 GPa, more preferably is higher than 5GPa, and most preferably is higher than 10 GPa, and the modulus ofrupture of the porous frame preferably is higher than 1 MPa, morepreferably is higher than 5 MPa, and most preferably is higher than 10MPa.

In addition, the porous frame should be able to scavenge any harmfulchemicals from the pellicle space that might be present duringpreparation of the photomask assembly and/or generated during thepreparation of the electronic chip. Such scavenging ability furtherreduces the number of potential defects in the photomask substratecaused by harmful chemicals. To achieve this scavenging, the innersurface of the porous frame has a surface area preferably larger than 10m²/g, more preferably larger than 25 m²/g, and most preferably largerthan 70 m²/g. In addition, the porous frame preferably is made of amaterial that can chemically and/or physically adsorb, or react with,one or more of the harmful chemicals identified above. The porous framepreferably can scavenge harmful chemicals present in an amountpreferably higher than an amount equivalent to 0.01 weight percent ofits own weight, and most preferably higher than an amount equivalent to0.05 weight percent of its own weight.

Above requirements can be fulfilled by preparing the porous frames usingglass and/or ceramic articles made of silica (SiO₂), fluorinated silica,zirconia (ZrO₂), alumina (Al₂O₃), SiO₂—Al₂O₃, SiO₂—B₂O₃, and mixturesthereof. The porous frame is prepared most preferably from silica orfluorinated silica articles having a purity above 96 weight percentsilica, because the CTE of these materials closely matches that ofsilica photomask substrates, fluorinated silica photomask substrates,silica pellicles, and fluorinated silica pellicles.

One example of a suitable porous frame material is commerciallyavailable porous glass, having a composition of about 96 weight percentsilica, sold by Corning Inc., of New York, under the trademark Vycor®7930. This glass has pores having an average size of 0.004 micrometer,and it has a CTE of 0.75 ppm/° C. This porous glass has a large innersurface area of 250 m²/g and thereby can effectively scavenge both watervapor and hydrocarbons from the pellicle space. The modulus of ruptureof the porous glass is 41.4 MPa. However, this porous glass containsimpurities, particularly Na, and also its CTE is higher than that ofmore than 99.9% pure synthetic silica. This porous glass can be used tomanufacture porous frames for the photomask assemblies having lessstringent material purity and CTE requirements, and it therefore isconsidered within the scope of this invention.

The porous frames can be prepared by cutting the porous articles usingsuitable machining processes, including laser machining, water jetmachining, and, most preferably, diamond tool machining and ultrasonicmilling. The porous articles can be cut directly into a frame shape asone piece. The frame also can be manufactured by cutting the porousarticles into thin rectangular bars and then attaching the bars togetherusing adhesives suitable for the photomask assemblies or using laserwelding. After these initial machining steps, the frames can be furthermachined and polished to meet the dimensional and flatnessspecifications of the photomask assembly industry.

The porous articles can be manufactured by variety of processes known tothe industry. Exemplary processes are described in the followingpublications: Kingery et al., “Introduction to Ceramics,” (John Wileyand Sons, 1976); King, “Ceramic Technology and Processing,” (NoyesPublications, 2002); Murata, “Handbook of Optical Fibers and Cables,”(Marcel Dekker, 1996); and Brinker et al., “Sol-Gel Science,” (AcademicPress, 1990). These processes include hand shaping, compacting, uniaxialpressing, hot pressing, hot isostatic pressing (HIP), injection molding,slip casting, tape casting, transfer molding, extrusion, chemical vapordeposition, and sol-gel. In an alternative process, Vycor® 7930 glass isprepared by etching a multicomponent glass, to increase the silicacontent of the glass to 96% and to cause the formation of pores havingan average size of 0.004 micrometer.

These processes also can use high pressures and temperatures to compactpowders and slurries. In these processes, dry or slightly damp powders,slurries, or colloidal solutions can be shaped into a plate, a rod, or aframe using a suitable mold or die. The articles thereby obtained may bedried, to remove volatile species such as water, alcohol, and acids usedin their preparation. These articles also can be fired, to removebinders or additives used in their preparation. The firing alsoincreases the mechanical strength of the article. The porous articlescan be cut into the frame or rectangular rod shape at any step of theseprocesses.

Sol-gel is a preferred process for preparing the porous frames, becauseof the process's flexibility in obtaining very high purity porousarticles having controllable permeability and controllable porestructure at a wide pore size and inner surface area range, and furtherhaving desired mechanical properties. In a preferred sol-gel process, asol is prepared using hydrolyzed silicon alkoxide and fine silicaparticles (fumed silica, e.g. Aerosil OX-50 manufactured by DegussaCo.), as described in published U.S. Patent Application 2002/0157419 A1to Ganguli et. al., which is incorporated by reference. The wet gelobtained by gelation of this sol then is dried using a sub-criticaldrying process described in U.S. Pat. No. 5,473,826 to Kirkbir et. al.This drying process minimizes shrinkage of the gel and decrease of poresize, and it also prevents cracking of the wet gel, which otherwise canoccur during drying. Because the gel does not significantly shrinkduring drying, large crack-free monolithic porous articles havingspecified pore structures can be easily obtained.

The dried gel then is partially densified at a temperature rangingbetween 650° C. to 1,260° C., more preferably between 1100° C. and1,200° C., and most preferably at about 1,180° C. in a controlledatmosphere of helium, nitrogen, oxygen, and their mixtures. Preferably,this atmosphere is a mixture of oxygen with either nitrogen or helium,the mixture having an oxygen concentration preferably between 3% and20%, and most preferably about 7%. The gel is heated to a partialdensification temperature preferably at a rate between 1° C./hr and 200°C./hr. The heating rate is more preferably between 10° C./hr and 100°C./hr, and most preferably about 15° C./hr. The gel is held at thistemperature for duration sufficient to provide sufficient strength anddevelop a desired pore structure, as discussed below.

The method of the present invention can further include a hydrocarbonburnout step, optionally performed after the drying step, in whichhydrocarbons and moisture adsorbed on the dry gel surface are removed byheating the dry gel to a temperature between 150° C. and 300° C., andmore preferably between 170° C. and 250° C., in a controlled atmosphereof oxygen and nitrogen. This atmosphere preferably contains 3% to 20%oxygen, and most preferably about 7% oxygen. The gel is heated to ahydrocarbon burnout temperature at a rate between 1° C./hr and 200°C./hr. More preferably, the heating rate is between 10° C./hr and 100°C./hr, and most preferably it is about 25° C./hr. The dwell time at thehydrocarbon burnout temperature is between 0.25 hour and 48 hours.Preferably, the dwell time is between 2 hours and 24 hours, and mostpreferably it is about 12 hours. The dry gel then is partially densifiedat a temperature ranging between 650° C. and 1,260° C., more preferablybetween 1100° C. and 1,200° C., and most preferably at about 1,180° C.,in a controlled atmosphere of helium, oxygen, nitrogen, or mixtures ofsuch gases. This atmosphere preferably is a mixture of oxygen and eithernitrogen or helium, the mixture having an oxygen concentration between3% and 20%, and most preferably about 7%. The gel is held at thistemperature for a duration sufficient to provide a desired strength andto develop a desired pore structure, as discussed below.

The method of the present invention can further include a halogenationstep, optionally performed after the steps of drying and hydrocarbonburnout described above, in which the gel is heated over a range oftemperatures between 500° C. and 1,200° C., more preferably between 650°C. and 1,050° C., and most preferably between 650° C. and 950° C. Duringthe halogenation step, a heating rate of between 10° C./hr and 200°C./hr, and most preferably about 25° C./hr, is used. Preferably, the gelis held at the halogenation temperature for duration of about 1 hour.The halogenation preferably is carried out at atmospheric pressure usinga mixture of a halogenation agent and an inert gas, such as helium ornitrogen. Examples of suitable halogenation agents are chlorine (Cl₂),thionyl chloride (SOCl₂), carbon tetrachloride (CCl₄), fluorine (F₂),silicon tetrafluoride (SiF₄), carbon ietrafluoride (CF₄), nitrogentrifluoride (NF₃), sulfur hexafluoride (SF₆), hydrochloric acid (HCl),hydrofluoric acid (HF), and mixtures of these agents. Preferredhalogenation agents are chlorine, thionyl chloride, and nitrogentrifluoride. The concentration of halogenation agent in the atmospherepreferably is between 0.1% and 100% and most preferably is about 33%.The halogenation process is carried out to remove hydroxyl (OH) ions andother impurities. The dry gel then is partially densified at atemperature ranging between 650° C. and 1,260° C., more preferablybetween 1100° C. and 1,200° C., and most preferably at about 1,180° C.,in a controlled atmosphere of helium, oxygen, nitrogen, or mixtures ofsuch gases. This atmosphere preferably is a mixture of oxygen and eithernitrogen or helium, the mixture having an oxygen concentration between3% and 20%, and most preferably about 7%. The gel is held at thistemperature for duration sufficient to provide a desired strength and todevelop a desired pore structure, as discussed below.

The method of the present invention can further include a step,optionally performed after the steps of drying, hydrocarbon burnout, andhalogenation, described above, in which the halogenated gel isoxygenated using methods known in the art to, remove halogen speciesremaining in the gel, and it is then re-halogenated. This step helps toremove additional impurities. The dry gel then is partially densified ata temperature ranging between 650° C. and 1,260° C., more preferablybetween 1100° C. and 1,200° C., and most preferably at about 1,180° C.,in a controlled atmosphere of helium, oxygen, nitrogen, or mixtures ofsuch gases. This atmosphere preferably is a mixture of oxygen and eithernitrogen or helium, the mixture having an oxygen concentration between3% and 20%, and most preferably about 7%. The gel is held at thistemperature for duration sufficient to provide a desired strength and todevelop a desired pore structure, as discussed below.

Based on the particular combination of temperature and time duration forthe specified partial densification step, porous silica glasses can beobtained exhibiting either lower strength but larger inner surface areaand higher permeability, or higher strength but smaller inner surfacearea and lower permeability. FIGS. 2 and 3 show, by way of example, theeffects of partial densification temperature and time on permeability,inner surface area, and the modulus of rupture. That is, the durationand temperature of the partial densification step are selected toachieve the proper balance of gas permeability and inner surface areawith mechanical strength, as required by a given application. Inparticular, the duration of this partial densification ranges between 5minutes and 48 hours, more preferably between 1 hour and 30 hours, andmost preferably about 4 hours. This provides a porous glass article thatis readily machined, but that is not yet brittle.

This porous article then is machined to the appropriate final dimensionsof the desired porous photomask frame. The photomask substrates arerectangular in shape, having standard dimensions of 12.5 cm×12.5 cm and15 cm×15 cm. The substrate thickness varies with application. For futureapplications, photomask substrates having dimensions of 22.5 cm×22.5 cmsize are being targeted. The dimensions of the patterned area of thephotomask substrate also vary with application. Thus, the frame's shapeand dimensions vary with the sizes of the photomask substrate size andthe patterned area. The porous article is machined to fulfill theseshape and dimensional requirements, as well as process requirements. Theporous article can be machined using conventional processes, such asdiamond turning, laser machining, water jet machining, and sonicmilling. The porous silica article can be machined to directly provide aframe. The frame also can be prepared by cutting the porous silicaarticle into rectangular bars and then attaching the bars together usingsuitable adhesives or using laser welding.

In a preferred method of the present invention, the dry gel is partiallydensified at an initial partial densification temperature, and it thenis machined to the porous frame shape. This takes advantage of thepotentially easier machining of the less-densified article. Thereafter,this intermediate porous silica article is heated to a final partialdensification temperature, to obtain the desired porous frame. Theinitial partial densification temperature is lower than the finalpartial densification temperature by between 50° C. and 300° C. Thefinal partial densification temperature preferably ranges between 650°C. and 1,260° C., more preferably ranges between 1,100° C. and 1,200°C., and most preferably is about 1,180° C. The gel is held at this finalpartial densification temperature for a duration sufficient to provide adesired strength and to develop a desired pore structure, depending onthe application, as discussed above.

In a more detailed feature of the method of the invention, the finalpartial densification step can be preceded by a step of subjecting thedry gel to an annealing temperature that is as much as about 300° C.lower than the initial partial densification temperature. This annealingstep removes stresses that might have developed in the article duringmachining. Finally, the annealed porous silica article is heated to thefinal partial densification temperature, to produce the desired porousframe. The final partial densification temperature preferably rangesbetween 650° C. and 1,260° C., and more preferably ranges between 1,100°C. and 1,200° C. The most preferable densification temperature is about1,180° C. The gel is held at this final partial densificationtemperature for a duration sufficient to provide a desired strength andto develop a desired pore structure, depending on the application, asdiscussed above.

In additional aspects of the method of the present invention, the stepof machining can be partly or completely avoided if the step ofpreparing the dry gel incorporates a molding process known in thesol-gel industry as a “net-shaping” or “near-net-shaping” process. Thesemolding processes can be used after experimental determination of thelevel of shrinkage of the particular gel caused by processing fromgelation to partial densification. The gel, formed by gelation of sol,assumes the inner dimensions of the mold. After the level of shrinkagecaused by processing has been experimentally determined, the finaldimensions of the gel after partial densification can be predicted. Inthis aspect of the method of the present invention, sols are cast intomolds having shapes and inner dimensions determined by theabove-described shrinkage experimentation, such that when the gelsobtained from such molds are dried and then directly (i.e., withoutmachining or intermediate machining) partially densified, they yieldarticles having dimensions substantially identical to that required,without the need for the step of machining of the articles.

The method of the present invention can be better understood by way ofthe following illustrative examples:

EXAMPLE 1

This example illustrates a preferred aspect of the method of the presentinvention, incorporating partial densification and machining. A sol wasprepared using the method disclosed in published U.S. Patent Application2002/0157419 A1 to Ganguli et. al. This sol was cast into a square moldhaving inner dimensions of about 26.7 cm×about 26.7 cm, to a height ofabout 18 mm. After gelation, the gel was subcritically dried accordingto techniques disclosed in U.S. Pat. No. 5,473,826 to Kirkbir et al. Thedried gel was free of any cracks.

To perform partial densification of the dry gel, the gel was placed inan electrically heated SiC furnace having a quartz enclosure. First,hydrocarbons and water vapor adsorbed on the dry gel surface wereremoved by heating the gel between 20° C. and 250° C., in an atmospherecontaining about 7% oxygen and about 93% nitrogen. In this step, the gelwas first heated from about 20° C. to about 170° C., at a heating rateof about 25° C./hr, and the gel then was held at about 170° C. for about5 hours. The gel then was further heated from about 170° C. to about250° C., at a heating rate of about 5° C./hr. The dry gel then was heldat about 250° C. for about 12 hours.

After the hydrocarbon removal step, the dry gel was heated in the sameatmosphere, at a rate of about 25° C./hr, to a temperature of about 650°C. The gel was then halogenated by heating it from about 650° C. toabout 1,050° C., at the same rate of about 25° C./hr, in an atmosphereof about 33% chlorine and about 67% helium. The gel was held at about1,050° C. for about 1 hour, for further halogenation. The atmospherethen was changed to be about 7% oxygen and about 93% helium, and the gelwas partially densified by heating the gel from about 1,050° C. to about1,180° C., at a heating rate of about 25° C./hr, and by holding the gelat about 1,180° C. for about 4 hours. The partially densified dry gelthen was cooled to room temperature and analyzed to characterize itsphysical properties.

The pore structure of the partially densified gel was characterized bynitrogen-adsorption equipment, model name Tristar, manufactured byMicromeritics, of Norcross, Ga., U.S.A. The surface area of this dry gelwas measured to be 29.8 m²/g, as shown in FIG. 2, and the average poresize was measured to be about 0.2 micrometer. The modulus of rupture ofthe partially densified gel was measured using a mechanical strengthanalyzer, Model No. 4202, manufactured by Instron, of Canton, Mass. Asshown in FIG. 2, the modulus of rupture was measured to be 39.1±6.7 MPa.The permeability of the partially densified gel was measured using apermeameter, Model No. G(E) 11142002-1135, manufactured by PMI (PorousMaterials Inc.), of Ithaca, N.Y. As shown in FIG. 3, the permeability tonitrogen was measured to be about 94.3 ml.mm/cm².min.MPa, and thepermeability to oxygen was measured to be about 76.5 ml.mm/cm².min.MPa.

The CTEs of both the partially densified gel and an aluminum articlewere measured over a temperature range of about 20° C. to about 290° C.,using a dilatometer, Model No. 1000D, manufactured by Orton, ofWesterville, Ohio. As shown in FIG. 4, the CTEs were measured to beabout 0.51 ppm/° C. for the porous silica article and about 25 ppm/° C.for the aluminum article. The CTE of a synthetic silica article,marketed by Corning under the trademark 7980, was measured to be about0.55 ppm/° C., using the same equipment and analysis conditions. Theseresults indicated that the CTE of the porous silica article prepared inthis Example is close to that of a silica photomask substrate.

The capability of the partially densified gel for scavenging of harmfulchemicals was measured at a temperature of about 20° C., using athermogravimetric analyzer, Model No. 7, manufactured by Perkin Elmer,of Boston, Mass. The results are shown in FIG. 5. In this measurement,dry nitrogen was first passed through a chamber containing the partiallydensified gel, for about 100 minutes, and this dry nitrogen stream thenwas moisturized with ethanol by diverting the stream through an ethanolbubbler before its entrance to the chamber. The weight of the partiallydensified gel increased due to adsorption of ethanol on its surface.This bubbling was stopped after about an additional 100 minutes, bybypassing the ethanol bubbler and again introducing the dry nitrogen tothe chamber. This caused partial desorption of the ethanol to the drynitrogen stream, resulting in a decrease in the weight of the gel. Thisdry nitrogen flow was continued for about 600 minutes. The amount ofweight gain at the end of the measurement was due to amount of ethanolirreversibly adsorbed on the gel surface, indicating that the gel'sscavenging capability was about 0.23 weight percent.

The porous plate thereby obtained was machined using a diamond toolmachine, Model No. VF-1, manufactured by HAAS, of Oxnard, Calif., toobtain a frame having an inner length of about 14.2 cm and an innerwidth of about 12.2 cm. The thickness of the frame was about 2 mm.

EXAMPLE 2

A porous frame was prepared and analyzed in the same manner as inExample 1, except that the gel was partially densified by holding it atabout 1140° C., instead of about 1180° C. Some of the analytical resultsare shown in FIGS. 2, 3 and 5. The resulting porous glass had a surfacearea measured to be about 76.3 m²/g land had an average pore sizemeasured to be about 0.09 micrometer. Its modulus of rupture wasmeasured to be about 20.7±2.0 MPa. Its permeability to nitrogen wasmeasured to be about 52.2 ml.mm/cm².min.Mpa, and its permeability tooxygen was measured to be about 46.0 ml.mm/cm².min.MPa. The CTE of thisporous glass was measured to be about 0.71 ppm/° C., and its scavengingcapability was measured to be about 0.56 weight percent after about 1000minutes of desorption.

Taken together, Examples 1 and 2 demonstrate that porous frames having awide range of pore structure and strength can be prepared by varying theduration of the partial heating. The large surface area gels, like oneobtained in Example 2, would be most useful for applications in whichhigh pore surface area is important and somewhat reduced strength isacceptable.

COMPARATIVE EXAMPLE 1

A porous frame was prepared and analyzed in the same manner as inExample 1, except that the gel was partially densified by holding it atabout 1220° C., instead of about 1180° C. Some of the analytical resultsare shown in FIGS. 2, 3 and 5. The resulting porous glass had a surfacearea measured to be about 18.4 m²/g and an average pore size measured tobe about 0.08 Micrometer. It had a modulus of rupture measured to beabout 137.5±38.6 MPa, and its permeability to nitrogen was measured tobe about 0.3 ml.mm/cm².min.MPa and its permeability to oxygen wasmeasured to be about 0.4 ml.mm/cm².min.MPa. The CTE of this porous glasswas measured to be about 0.58 ppm/° C., and its scavenging capabilitywas measured to be about 0.12 weight percent after about 250 minutes ofdesorption.

COMPARATIVE EXAMPLE 2

A porous frame was prepared and analyzed in the same manner as inExample 1, except that the gel was partially densified by holding it atabout 1260° C., instead of about 1180° C. Some of the analytical resultsare shown in FIGS. 2, 3 and 5. The resulting porous glass had a surfacearea measured to be about 0.3 m²/g and an average pore size measured tobe about 0.03 micrometer. It had a modulus of rupture measured to beabout 121.7±20.0 MPa, and its permeability to nitrogen was measured tobe about 0.1 ml.mm/cm².min.MPa and its permeability to oxygen wasmeasured to be about 0.5 ml.min/cm².min.MPa. The CTE of this porousglass was measured to be about 0.6 ppm/° C., and its scavengingcapability was determined to be negligible after about 250 minutes ofdesorption.

Taken together, Comparative Examples 1 and 2 demonstrate that if the drygels are partially densified at a temperature of about 1200° C., forlonger than about 4 hours, the permeability of the porous articledecreases to an unacceptable level, at which the purging of the pelliclespace from the harmful chemicals cannot be achieved within a reasonableprocessing time.

Although the invention has been described in detail with reference onlyto the preferred articles and methods described above, those of ordinaryskill in the art will appreciate that various modifications can be madewithout departing from the scope of the invention. Examples of suchmodifications include changes to the porous frame material,permeability, pore size, scavenging capability, and CTE, as well aschanges to the steps of hydrocarbon burnout, halogenation, oxygenation,partial densification, and machining.

1-24. (canceled)
 25. A method for making a photomask assembly,comprising: providing a photomask substrate, a porous frame, and apellicle, wherein the porous frame has a gas permeability to oxygen ornitrogen higher than about 10 ml.mm/cm².min.MPa, an average pore sizebetween 0.001 micrometer and 10 micrometers, and a coefficient ofthermal expansion between 0.01 ppm/° C. and 10 ppm/° C.; and attachingtogether the photomask substrate, the porous frame, and the pellicle, toform a photomask assembly.
 26. A method as defined in claim 25, whereinproviding a porous frame comprises: (a) preparing a gel by a sol-gelprocess, (b) drying the gel, and (c) partially densifying the dry gel.27. A method as defined in claim 26, and further comprising machiningthe densified dry gel to form the porous frame.
 28. A method as definedin claim 26, and further comprising machining the densified dry gel toform rectangular bars and welding the bars to form the porous frame. 29.A method as defined in claim 26, and further comprising machining thedensified dry gel using a process selected from the group consisting ofdiamond tool machining, ultrasonic milling, laser machining, and waterjet machining.
 30. A method as defined in claim 26, and furthercomprising machining the densified dry gel using a process selected fromthe group consisting of diamond tool machining and ultrasonic milling.31. A method as defined in claim 26, wherein preparing a gel comprisesshaping the gel in a mold having dimensions such that when the gel issubsequently dried and partially densified, the frame will be configuredto have desired dimensions without the need for machining.
 32. A methodas defined in claim 26, and further comprising machining the densifieddry gel to less than 20-micrometer surface flatness.
 33. A method asdefined in claim 26, wherein the dry gel comprises silica.
 34. A methodas defined in claim 26, wherein the gel comprises silicon alkoxide andfumed silica.
 35. A method as defined in claim 26, wherein partiallydensifying comprises partially densifying the dry gel at a prescribedpartial densification temperature in an atmosphere comprising helium,nitrogen, oxygen, or mixtures thereof.
 36. A method as defined in claim35, wherein partially densifying occurs at a partial densificationtemperature within a range of 650° C. to 1260° C.
 37. A method asdefined in claim 35, wherein partially densifying occurs at a partialdensification temperature within a range of 1100° C. to 1200° C.
 38. Amethod as defined in claim 35, wherein partially densifying occurs at apartial densification temperature of about 1180° C.
 39. A method asdefined in claim 35, wherein partially densifying comprises heating thedry gel to the prescribed partial densification temperature at a ratebetween 1° C./hr and 200° C./hr.
 40. A method as defined in claim 35,wherein partially densifying comprises heating the dry gel to theprescribed partial densification temperature at a rate between 10° C./hrand 100° C./hr.
 41. A method as defined in claim 35, wherein partiallydensifying comprises heating the dry gel to the prescribed partialdensification temperature at a rate of about 15° C./hr.
 42. A method asdefined in claim 35, wherein partially densifying comprises maintainingthe dry gel at the prescribed partial densification temperature for aduration in a range of 1 hour to 100 hours.
 43. A method as defined inclaim 35, wherein partially densifying comprises maintaining the dry gelat the prescribed partial densification temperature for a duration in arange of 1 hour to 30 hours.
 44. A method as defined in claim 35,wherein partially densifying comprises maintaining the dry gel at theprescribed partial densification temperature for a duration of about 4hours.
 45. A method as defined in claim 35, wherein the atmosphereconsists essentially of a mixture of oxygen and nitrogen or helium, themixture having an oxygen concentration between 3% and 20%.
 46. A methodas defined in claim 35, wherein the atmosphere consists essentially of amixture of oxygen and nitrogen or helium, the mixture having an oxygenconcentration of about 7%.
 47. A method as defined in claim 26, andfurther comprising removing hydrocarbons from the dry gel by heating thedry gel at a temperature between 150° C. and 300° C.
 48. A method asdefined in claim 47, and further comprising halogenating the dry gelusing a halogenation agent at a temperature between 650° C. and 1,200°C., after the step of removing hydrocarbons.
 49. A method as defined inclaim 48, and further comprising: (a) oxygenating the dry gel after thestep of halogenating, and (b) re-halogenating the dry gel after the stepof oxygenating.
 50. A method as defined in claim 26, wherein the step ofpartially densifying the dry gel further comprises: partially densifyingthe dry gel at a prescribed initial partial densification temperature;machining the partially densified dry gel to a desired porous frameshape; and partially densifying the porous frame at a prescribed finalpartial densification temperature, wherein the final partialdensification temperature is greater than the initial partialdensification temperature by between about 50° C. and about 300° C. 51.A method as defined in claim 50, wherein the prescribed final partialdensification temperature is in a range of 650° C. and 1,260° C.
 52. Amethod as defined in claim 50, wherein the prescribed final partialdensification temperature is in a range of 1,100° C. and 1,200° C.
 53. Amethod as defined in claim 50, wherein the prescribed final partialdensification temperature is about 1,180° C.
 54. A method as defined inclaim 50, wherein partially densifying the dry gel comprises: partiallydensifying the dry gel at the prescribed initial partial densificationtemperature; machining the partially densified dry gel after the step ofpartially densifying the dry gel, to produce a desired porous frameshape; annealing the machined dry gel, at an annealing temperature thatranges between the initial partial densification temperature and about300° C. lower than the initial partial densification temperature; andpartially densifying the annealed dry gel at a prescribed final partialdensification temperature, wherein the final partial densificationtemperature is higher than the initial partial densification temperatureby between about 50° C. and about 300° C.
 55. A method of making aporous silica frame suitable for use in a photomask assembly,comprising: preparing a dry gel comprising more than 99.9% silica, usinga sol-gel method; halogenating the dry gel by heating the dry gel fromabout 650° C. to about 1,050° C., at a heating rate of about 25° C./hrin an atmosphere of about 33% chlorine and about 67% helium, andmaintaining the dry gel at about 1,050° C., for a duration of about 1hour in the atmosphere; partially densifying the halogenated dry gel byheating the halogenated dry gel from about 1,050° C. to about 1,180° C.,at a heating rate of about 25° C./hr in an atmosphere of about 7% oxygenand about 93% helium, and maintaining the halogenated dry gel at about1,180° C. for about 4 hours; and machining the partially densified drygel into a desired frame shape having a flatness of less than 1micrometer.